![\"Extrusion](\"http:\/\/www.polymers.com.ua\/image\/data\/NEWS\/15.07.15\/23\/1\/5154465.jpg\" "\\"Extrusion")
<\/a><\/h1>\nUnder the impact of the accelerated secondary electrons and gamma radiation is breaking of chemical bonds, the formation of free radicals which recombine to create cross-links between the macromolecules.
\nProduction pipe according to this process is divided into two independent steps of: making tubes on conventional pipe lines and subsequent processing of high-energy mainly accelerators elektronov.Proizvodstvo small pipe diameters and wall thicknesses, respectively, causes technological difficulties, without removing the technical problem , organizational and economic nature relating to the establishment and operation of radiation equipment sshivki.Uvelichenie wall thickness of more than 3-4 mm requires special measures. Pipe brand of high density polyethylene to achieve the desired degree of crosslinking required irradiation dose 15-20 Mrad. Thus the absorbed dose of 1 Mrad warms up polyethylene on 5O C. Avoid overheating possible in two ways: – to carry out multiple pipes run through the accelerator, which requires additional equipment costs and reduces the efficiency of the process;<\/p>\n
– Use of polyethylene compositions containing a sensitizing additive lowering the required dose of irradiation of 10 Mrad, in turn, substantially increases the cost of the feedstock.
\nTo obtain a uniform crosslinking pipe as the thickness and perimeter, the accelerator should have specific scanning electron beams. For pipe wall thickness of 10 mm require accelerators with energies of 3 MeV to 15 mm – 5 MeV with a power of 50-100 kW. The cost of such accelerators is 1.5 – 2 mln. US dollars. We should not forget the need for expensive protection from ionizing radiation of the accelerator. When chemical crosslinking of the hydrogen atoms in the molecules of polyethylene are replaced by chemicals. One such material is a silane. Therefore, a chemical called silane crosslinking. Polyethylene pipes at the outlet from the extruder passes through a bath of silane, the crosslinking process is from two surfaces – the outer and inner deep wall of the pipe. In this case, both surfaces of a high percentage of cross-linking, and in the middle of the thickness of the tube is the smallest.
\nThe average percentage of cross-linking is about 75%. Such material is usually denoted by REH-B. Crosslinking of polyethylene molecules occurs by grafting to polyethylen-Lena silanol groups of compounds and the subsequent treatment with water-tion wi silaksanovyh svyazey.Suschestvuyut two silanol production process crosslinked pipes: – two-step process wherein the first step is carried out by grafting polyethylene silanol compounds (made in plants producing polyethylene), and the second stage representing a conventional extrusion, this polyethylene is injected crosslinking catalyst – in a one step process (modification of which differ slightly from each other) in the extruder is fed immediately polyethylene, silane activator grafting and crosslinking catalyst.<\/p>\n
In both cases, for the production of pipes uses the standard line pipe on the basis of single-screw extruders. After extrusion, in which the degree of crosslinking is not more than 15%, it is necessary to perform additional treatment with water at elevated temperature to achieve a degree of crosslinking of the prescribed standard. It should be borne in mind that this process is quite long, for example, for tubes with a wall thickness of 10 mm at a temperature of 80 \u00b0 C the processing time is about 12 sutok.Vtoroy method for chemical crosslinking – nitrogen radicals crosslinking. Crosslinked polyethylene in this manner is denoted REH-D. This technique is now almost never used because of the low adaptability. The third method of chemical cross-linking – crosslinking peroxides. Features of this method is that the polyethylene and crosslinking initiator – Peroxide – preliminarily uniformly mixed. Crosslinking is performed under high pressure into a molten state. With this method achieves a high percentage of crosslinking equal to an average of 85%. The properties of the material are the same at each point regardless of the material thickness. This is denoted polyethylene REH-A. Crosslinking occurs due to the formation of active radicals under the action of peroxide introduced into the polyethylene in the preparation stage of the composition. Two methods of production of pipes – with cross-linking of polyethylene during the extrusion of the polymer flow in the forming tool, – with a crosslinking after extrusion of polyethylene under the influence of infrared radiation with wavelengths selectively acting on peroxide and causing its activation, without significant heating and softening of polyethylene.
\nIn both cases used for the production of line pipe with a standard set of units, The difference is that in the former case, a piston extruder with an elongate forming tool, in the second conventional screw extruder, after which the heater is installed in a similar manner pech.Izgotovlennye pipes have the following characteristics .Alyuminievaya foil does not affect the operating parameters of the pipe – the operating pressure and temperature. The fluid inside the pipe is moved not aluminum, but on the plastic tube, and therefore it is the properties of XLPE determined operating parameters. The main task of the aluminum layer – the creation of a diffusion barrier to prevent the penetration of oxygen from the atmosphere into the tube. Other functions performed by the aluminum layer, – partial compensation of thermal expansion of the polymer tube. Since the coefficient of linear thermal expansion of the polymers is 10-12 times higher than that of steel, that when operating in hot water and heating are elongated plastic tubes. However, this fact does not have a serious impact on the ease of use and installation of plastic pipes. Thus, the foreign experience of building completely eliminates the open laying of internal sanitation systems – pipes are laid in special ducts and channels, giving easy access to them, as well hides from the eyes of “bending” of pipes due to thermal expansion, which does not affect their performance, but it seems unaesthetic when laying pipes on the wall surface. The problem of linear expansion of plastic pipe in the riser can be easily solved by using special compensators. Therefore, the compensation function of the linear extension, additionally performed with aluminum foil, make sense only if the open piping in heating systems, where the length of the pipelines is large enough. Well, in cold water and warm floors (there are pipes in the concrete monolith) compensation at all bespolezna.Pomimo positive qualities, used in the construction of pipe and aluminum foil has a negative side. First of all, the more complex the design, the higher the probability of its failure. Furthermore, since the coefficient of linear thermal expansion of the “cross-linked” polyethylene and aluminum are different, during operation the various layers of the pipe extend in different ways. As a result, there are significant design stresses which must compensate the adhesive layer. Unfortunately, not all manufacturers fully withstand all the requirements to comply with the production technology and quality materials. As a result – in the tube bundle during operation. Well, if you talk about the resistance of metal-polymer pipes to freeze, here they are even more vulnerable position. Upon expansion the water inside the tube due to freezing, the deformation of aluminum foil and the risk of rupture of the weld.<\/p>\n
Chemical properties
\nMetal pipes are resistant to various chemical solutions.<\/p>\n
Saving forms
\nAfter bending metal pipes retain the desired shape, which facilitates and speeds up the assembly of fittings and further work with the pipe.<\/p>\n
Resistance to wear
\nThe inner layer of plastic pipes made of high-strength cross-linked polyethylene. This provides a practically complete absence of wear, even at high flow rates. The high strength allows for water supply pipes with high internal pressure. Also plastic pipes are resistant to multiple, sudden changes in pressure and temperature.<\/p>\n
The coefficient of expansion
\nThanks to the aluminum layer linear expansion coefficient of plastic pipes is 0.025 mm \/ (IBSC).<\/p>\n
Heat resistance
\nIt allows you to use the pipes for the installation of hot water.<\/p>\n
Electrical safety.
\nThe special design allows the fitting to interrupt the electrical contact and put a barrier in the way of premature failure of the system due to exposure to electrical fields.<\/p>\n
Environmentally friendly
\nMetal pipes are not contraindicated for use in all types of pipelines for drinking vody.Blagodarya that the outer layer of the pipe is also cross-linked polyethylene or polypropylene, there is no need to protect the pipe from the outside against corrosion, and there is no need to paint it.<\/p>\n
Let us analyze separately the comparative advantages and disadvantages of different types of metal-polymer tubes.
\nPositive aspects of the production and use of PEX-a:
\nmore reliable achievement of uniformity of cross-linking;
\nhigher degree of crosslinking (80%), PEX-c is usually not much higher than 60%;
\nsignificantly higher flexibility as compared with other types of PEX, especially 63-160 mm diameter pipe with large wall thickness;
\nrelatively low cost raw material (compared with PEX-c);
\nmost cheap and simple manufacturing equipment;
\nthe maximum experience of using pipes (the method of crosslinking Engel was historically the first of more than 30 years ago, and the pipes have practically proven lifespan of over 25 years);
\nAll European companies producing pre-insulated pipes for external systems, hot water and heating (UPONOR-WIRSBO, REHAU, BRUGG) exclusively use PEX-a.
\nDisadvantages of PEX-a are purely technological in nature:
\nlow productivity;
\nthe need for the development of KNOW-HOW, primarily associated with a complex manual process forms processing tool.
\nPositive aspects of PEX-c. \u00b7 High-speed production of pipe extrusion stage; \u00b7 standard technological pipe lines used in the production; \u00b7 the ability to achieve improved physical and mechanical properties of the pipe on new kinds of materials (TUX-100). \u00b7 Lacks PEX-c.
\nHigh cost of raw material (twice PEX-a);
\nthe need for additional manufacturing steps – linking tubes by exposure to elevated temperatures and moisture;
\nproblems crosslinking pipes of large thickness (time uniformity);
\nLowest flexible pipe (high density) – the flip side of high physical and mechanical properties;
\nhygienic properties of pipes resulting from use of the silanol additive – questionable.
\nPositive aspects of PEX-a.
\nperformance of the first stage of production (tube extrusion);
\nstandard production lines used in the first stage of production;
\nlow cost of raw materials in the process without the use of sensitizing agents.<\/p>\n
Disadvantages of PEX-a.
\nlow degree of crosslinking;
\nthe need for additional capital-intensive and responsible production stage – crosslinking irradiation tubes;
\nproblems crosslinking pipes of large thickness (time uniformity);
\ncapital investments are effective only at very high production program.<\/p>\n<\/div>\n
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Under the impact of the accelerated secondary electrons and gamma radiation is breaking of chemical bonds, the formation of free radicals which recombine to create cross-links between the macromolecules. Production pipe according to this process is divided into two independent steps of: making tubes on conventional pipe lines and subsequent processing of high-energy mainly accelerators […]<\/p>\n